Marc Delvaux is an ex-cop. Since his wife died, he lives alone, confined to the ground floor of their large house due to the after-effects of an old leg injury. He's visited by Bernard Lerieux, a renowned attorney-at-law in Lyon, with whom he shares a love of chess. Bernard tells him he's working an "explosive" case. The outcome of the case depends on a surprise witness only he knows about. This witness must be kept a secret until the trial - and Bernard asks Marc to hide her at his house. Marc agrees. That evening, 28-year-old Emma shows up. She'll occupy the upper floor, where no one ever goes. But soon - unaware to Marc - the young woman begins to act strangely - She slips a sleeping pill into his coffee, pulls out one of his hairs, photographs the pages of a text he's writing - is she really at Marc's house by chance? Especially as she tries to isolate him even more. But Marc is not totally fooled, despite not fully knowing her reasons.